Generally, most active medical devices are designed so as to allow data exchanges with a “programmer,” an external apparatus used to check the parameter settings of the device, read information recorded thereby or write information thereto, or update the internal software for driving the device.
Such a data exchange between the medical device and the programmer is operated by telemetry, i.e. a technique of remote transmission of information, without galvanic contact.
Up to now, telemetry for implanted devices used to be essentially performed by magnetic coupling between a coil of the implanted device and a coil of the programmer (or programming head), a technique known as the “induction technique”. However, that technique, due to the very short range of inductive coupling, has the shortcoming of requiring the use of a “telemetry head” or wand linked to the programmer and containing a coil that an operator puts in the proximity of the site where the device is implanted.
Recently, another technique has been proposed implementing a non-galvanic coupling using the two components of an electromagnetic wave produced by transmitter/receiver circuits operating within the radiofrequency (RF) domain, typically with frequencies ranging around several hundreds of Megahertz. That technique, known as “RF telemetry”, allows one to program or interrogate implanted devices from distances greater than 3 m, and therefore allows the exchange of data without the need for using a telemetry head, and even without the intervention of an external operator. A device comprising means of RF telemetry and associated programmer, are, for example, described in U.S. Pat. No. 6,868,288 (Thompson).
A satisfactory functioning of RF telemetry circuits implies an efficient elimination of RF parasites that are likely to produce interference and disturb data transmission. Indeed, differently from induction techniques, which present a good immunity against parasites, the RF signal reception is strongly disturbed by the electromagnetic environment, notably radio signals, TV and mobile phone signals, and also the numerous industrial parasites likely to be produced within the immediate surroundings of a patient implanted with a device.
RF telemetry circuits therefore require the use of very efficient band-pass filters, presenting a very abrupt band rejection characteristic. Acoustic wave filters present such characteristics, and Thomson U.S. Pat. No. 6,868,288, cited above, precisely proposes to use, in the RF telemetry circuit of an implantable device, a Surface Acoustic Wave resonator (SAW) or a Thin Film Bulk Acoustic Resonator (FBAR). Such resonators are indeed well known for their characteristics presenting a very high selectivity, and for their use in the realization of very efficient band-pass filters.
Those SAW or FBAR resonator filters however present a few shortcomings, especially when they are used in implanted devices. Indeed, SAW resonators use, by principle, the surface propagation of an acoustic wave, which correlatively implies a relatively large component size. This shortcoming is particularly embarrassing when it comes to implanted devices, which, as it can be easily understood, require an advanced downsizing of electronic circuits, due to the small available space within their cases.
Also, from a technological point of view, these SAW resonators are only available as discrete components that must be bonded onto the electronic circuit of the implant, with consequent shortcomings, notable in terms of additional steps as part of manufacturing processes and a higher cost.
Finally, from the electrical point of view, SAW resonators present an excellent selectivity, but introduce significant insertion losses in the circuits in which they are used, consequently affecting the sensitivity of the telemetry receiver.
Differently from SAW resonators, FBAR resonators present a much smaller size and lower insertion losses. However, these FBAR resonators are more difficult to make, for they require micro-machining of a very thin movable membrane, likely to become resonant (one will describe more in details the structure of FBAR resonators by reference to FIGS. 2a and 2b). That micro-machining is difficult to implement, notably as part of a collective process, which introduces much scrap during manufacturing of the components.
Besides, the movement of the movable membrane implies that a free space exists below and above it. For that reason, if any particles (such as dust) were to deposit above the membrane during or after the manufacturing process, the resonance frequency thereof would be modified, and it would be necessary to re-tune the filter. So as to palliate this shortcoming, once the component has been made and adjusted in terms of frequency, it has to be enclosed in a hermetic volume, with the forming of an additional layer, or bonding of a closure cap for the component.
From an industrial point of view, though it is theoretically possible to integrate the FBAR resonator along the manufacturing process of the circuit it is associated with, such an integration is practically difficult to implement with a satisfactory yield, which is the reason why the FBAR resonators available nowadays are only available in the form of discrete components. The FBAR component must therefore be bonded onto the electronic circuit or onto the hybrid board through an additional process such as wire-bonding or flip-chip, which, similarly to the case of SAW resonators, dramatically increases manufacturing cost and reduces electrical performances due to the presence of wires, conductive pathways, etc.
Apart from the difficulty to find and sustain the tuning of an FBAR resonator, this latter presents a rejection characteristic less abrupt than that of a SAW resonator. Thus, to obtain a satisfactory rejection level, it is necessary to combine together a plurality of FBAR resonators. Such a multiplication of components consequently increases cost, volume and difficulties for developing the final circuit.